1. Field of the Invention
The invention relates to a heating or cooling roller comprising a stationary axle, at least one hydrostatic or hydrodynamic bearing element to support a rotary roller shell on the stationary axle, and a heatable or coolable heat transfer medium which flows through an annular gap between the stationary axle and the rotary roller shell.
2. Description of the Prior Art
Such rollers are used to produce and process webs of material, especially paper webs, and contribute greatly to the quality of the material webs to be produced or processed. Thus, among otherthings, it may be desirable to heat or cool the roller shell, hence directly affecting the material to be treated.
There are already heatable rollers with a rotary external roller shell and a stationary axle throughout the entire length.
Heating of the roller can take place internally, i.e. by means of a fluid heat transfer medium, especially thermal oil, which is fed into the gap between the axle and roller shell, or externally, i.e. especially by inductive heating.
The disadvantage of internally heated rollers is that the heat must flow through the roller shell from the inside to the outside; the shell thickness, however, is relatively great due to the required mechanical strength and therefore hinders heat flow. If relatively large amounts of heat are supplied to or removed from the roller body, a large temperature gradient from the inside to the outside is necessarily created.
To maintain a certain surface temperature, the temperature of the heat transfer medium must be raised or lowered accordingly.
The distortion of the roller body induced by the large temperature gradient causes, especially in the edge region of the roller, a deformation (the so-called "Oxbow Effect") so that only material webs of low quality can be produced there.
In the roller known from U.S. Pat. No. 4,459,726, the heat transfer medium is supplied and removed by a special sealing head and a special roller bearing with three track rings. This design is, however, extremely expensive and cannot be economically realised under practical conditions. Therefore these rollers have not been used to date.